Processes for bonding substrates with adhesives solid at room temperature are known, thus, the hotmelt adhesives solid at room temperature are first heated until they become tacky and are then applied in the form of a melt to the substrates to be bonded. After the substrates have been fitted together, the hotmelt adhesives set physically and solidify on cooling through crystallization or an increase in viscosity. Raw materials for such hotmelt adhesives are, for example, polyethylene vinyl acetate, polyamide, polyester and polyurethane. One such PU hotmelt adhesive is described in WO 94/13726. This document claims a water-soluble, high molecular weight nonionic partly crystalline polyurethane as the basis for a hotmelt adhesive. The polyurethane is characterized by the following structural units:
a) --O(--CH.sub.2 --CH.sub.2 --O).sub.n --, PA1 b) --CO--NH--X--NH--CO--, PA1 c) --O--Y--O--, PA1 a) at least one aliphatic or aromatic diisocyanate, more particularly MDI, TDI, HDI, IPDI and, above all, TMXDI, PA1 b) at least one crystallizing diol, more particularly from the following group: PA1 c) optionally at least one diol capable of forming ions, more particularly carboxylate, sulfate or ammonium ions, PA1 d) optionally at least one polyol with a functionality of 3 or more, such as glycerol and TMP, and PA1 e) optionally at least one hydrophobic diol, more particularly from the following group: PA1 the ratio of the isocyanate groups to the hydroxyl groups being variable from 0.5 to 1.2:1 and more particularly from 0.7 to 1:1. PA1 a) 0 to 50% by weight and more particularly 0 to 20% by weight of at least one crystallinity-modifying additive, more particularly from the following group: salts of aromatic and aliphatic carboxylic acids (for example Ca stearate), wax, polyacrylate, polyethylene, polyvinyl acetate, polyamide, polyurethane and polyvinyl chloride and also polyester or polyurethane where the binder is a polyurethane or a polyester, PA1 b) 0 to 20% by weight, more particularly 0 to 10% by weight and preferably 0.1 to 5% by weight of at least one fine-particle water-insoluble pigment or filler, more particularly from the following group: alkali metal stearate, graphite, talcum, TiO.sub.2, highly disperse silica (Aerosil), bentonite, wollastonite, chalk, magnesium oxide and glass fibers, PA1 c) 0 to 30% by weight and more particularly 0 to 10% by weight of at least one non-volatile plasticizer, above all from the group of phthalates, sebacates, phosphates, for example: diphenyl phthalate, benzylbutylphthalate, trioctyl phosphate and n-ethyl-o,p-toluene sulfonamide, PA1 d) 0 to 5% by weight and more particularly 0 to 2% by weight of at least one of the following additives: antioxidants, preservatives and dyes, PA1 e) water and PA1 f) 0 to 30% by weight and more particularly 0 to 10% by weight of at least one tackifier, above all from the following group: terpene/phenol resin, rosin/glycerol ester, polycyclopentadiene resin, hydrocarbon resin and methyl styrene/styrene copolymer. PA1 It requires little or no packaging for normal ambient conditions (20.degree. C./50% relative air humidity). PA1 Its composition does not have to be shown on a label. PA1 It is non-inflammable. PA1 It is mechanically activated by rubbing under light pressure. PA1 It is non-stringing. PA1 The bond can be broken by heating. PA1 Paper does not curl after application of the adhesive. PA1 The setting rate of the adhesive is very high: the adhesive feels dry (and non-tacky) only a few seconds after application. PA1 The adhesive is easy to produce by virtue of its rapid recrystallization and its low melt viscosity.
where n=8 to 500 and, more particularly, 20 to 300, PA2 where X is an aliphatic or cycloaliphatic residue, more particularly a residue of m-tetramethyl xylene diisocyanate (TMXDI), and PA2 where Y is a hydrophobic residue, more particularly either PA2 (--CH.sub.2 --CH(CH.sub.3)--O).sub.m --CH.sub.2 --CH(CH.sub.3)--, PA2 (--CH.sub.2 --CH(C.sub.2 H.sub.5)--O).sub.m --CH.sub.2 --CH(C.sub.2 H.sub.5)-- and PA2 (--CH.sub.2 --CH.sub.2 --CH.sub.2 --CH.sub.2 --O).sub.m --CH.sub.2 --CH.sub.2 --CH.sub.2 --CH.sub.2 -- PA2 where m=8 to 500 and, more particularly, 20 to 300, or an alkylene or cycloalkylene group containing 2 to 44 carbon atoms and, more particularly, 6 to 36 carbon atoms, PA2 c) making up 0 to 40% by weight, more particularly 2 to 30% by weight and preferably 5 to 25% by weight, based on a)+c) in the polyurethane. PA2 polyethylene glycol with an average molecular weight (number average) in the range from 200 to 40,000, more particularly in the range from 1,500 to 15,000 and preferably in the range from 4,000 to 8,000, PA2 polytetrahydrofuran with a molecular weight in the range from 200 to 4,000 and more particularly in the range from 1,000 to 3,000, PA2 a copolymer of ethylene oxide and propylene oxide with a molecular weight in the range from 200 to 40,000 and preferably in the range from 400 to 10,000, the copolymer best being a block copolymer of the PEG/PPG/PEG type with a PEG content of 10 to 80% and, optionally, PA2 polyester diol, more particularly a polycaprolactone diol with a molecular weight in the range from 200 to 50,000 and more particularly in the range from 200 to 5,000, PA2 polypropylene glycol with a molecular weight in the range from 200 to 4,000 and more particularly in the range from 500 to 2,000 and PA2 an alkanediol containing 1 to 100 carbon atoms, more particularly 2 to 50 carbon atoms and preferably 5 to 30 carbon atoms,
Hotmelt adhesives have the general disadvantage that a heat source is required for melting.
Adhesives solid at room temperature which are suitable for bonding without melting, for example adhesive sticks, are also known. In order to bond substrates with an adhesive stick, the adhesive stick is simply drawn over the substrate to be bonded and the substrate thus coated with adhesive is fitted together with the other substrate. The adhesive which is tacky at room temperature sets by evaporation of the solvent or the water or by diffusion of the solvent or the water into the substrate.
One such adhesive stick is described in EP 405 329. The dimensionally stable, soft-rubbing adhesive stick consists of an aqueous formulation of a polyurethane as binder, a soap gel as the shaping gel-forming component and, if desired, auxiliaries. The polyurethane is a reaction product of a polyol or polyol mixture, an isocyanate component with a functionality of 2 or more, a component capable of salt formation in alkaline aqueous solution and/or a nonionic hydrophilic modifier and, if desired, a chain-extending agent. In Example 1d, a polyurethane was prepared from 29.7 parts by weight of isophorone diisocyanate, 100 parts by weight of polyethylene propylene glycol with an EO content of 10% and a molecular weight of 2,000, 6.8 parts by weight of dimethylol propionic acid and 2.2 parts by weight of NaOH. In the acetone process, an aqueous dispersion with a solids content of 36% by weight was prepared. Finally, the adhesive was produced from 82 parts by weight of this PU dispersion and 2 parts by weight of water, 7 parts by weight of glycerol, 3 parts by weight of PPG 600, 3 parts by weight of sodium palmitate and 3 parts by weight of sodium stearate. The adhesive had a melt viscosity of 2.4 Pas at 60.degree. C. An adhesive stick of the type in question has the disadvantage that it requires a sealed pack, otherwise it would be in danger of drying out and its properties would be adversely affected. In addition, paper curls under the effect of the high water content of the adhesive.
These disadvantages are avoided in an adhesive stick which consists of a solid adhesive component and a microencapsulated solvent (cf. GB 995,524). Unfortunately, this adhesive stick has the disadvantage that, after use, a skin forms over its surface and has to be laboriously removed before the adhesive stick can be used again.
The disadvantages of a water-based adhesive stick are also avoided by the adhesive stick based on wax, polypropylene and rosin according to DE 20 22 464. The stick is activated by frictional heat, the uppermost adhesive layer melting. On cooling, the stick sets almost instantaneously so that correction is no longer possible. In addition, the handling of this known stick is complicated by stringing. Finally, the presence of rosin has to be indicated on the label.
Against the background of this prior art, the problem addressed by the present invention was to provide a bonding process and an adhesive suitable therefor which would not have any of these disadvantages and would be easy to handle. This would include in particular little or no packaging, application under light pressure, a composition that would not have to be shown on a label and simple elimination of the bond as when required. The adhesive would be particularly suitable for paper and paperboard.